Aircraft ejection seats generally comprise a seat pan mounted on a beam or frame adapted to be ejected with the seat pan bodily along a predetermined path from the aircraft, ejection means being provided to project the ejection seat from the aircraft in a predetermined direction relative to the aircraft, when required in an emergency. Ejection of the seat from the aircraft is generally effected by means of an ejection catapult comprising two or more telescopically cooperating parts adapted to be thrust axially apart by propulsion gases generated by the firing of at least one combustible cartridge. The ejection catapult operates between the seat frame and a fixed part of the aircraft, being located in such a position as to exert the thrust in the direction that it is desired that the ejection seat should travel as it moves from the aircraft.
When an ejection seat/airman combination is ejected from an aircraft in an emergency it is desirable that this combination should attain a high velocity in a predetermined direction in as short a time as possible consistent with the application of physiologically acceptable acceleration values to the airman. It is important that the trajectory of the seat/aircraft combination should be such that the combination will adequately clear all parts of the aircraft in any flight condition. It is also important that the combination should have sufficient time for the deployment and effective operation of the airman's parachute.
The ejection catapult of an ejection seat as described above can only produce an effective accelerating thrust during the time that the telescopically cooperating parts of the catapult are interengaged. This means that the ejection catapult must produce a very high short term acceleration in order to achieve a high seat/airman combination velocity. Accordingly, the maximum attainable velocity is limited by the maximum acceleration value that can safely be applied to the airman by the operation of the ejection catapult.
The combustible cartridge or propellant grain is designed to provide a required acceleration over ambient temperatures ranging from -65.degree. F. to 160.degree. F. It is known that the burning rate of many solid propellants varies considerably with the temperature. Thus, if conditions are adjusted for 70.degree. F., then at -40.degree. F. combustion may be slowed to an extent retarding acceleration so that the seat/airman combination may not be cleared above a tail fin or other part of an airplane. Conversely, if the conditions are adjusted for 70.degree. F., then at +140.degree. F., acceleration may be too high and likely to cause the airman to black out or suffer spinal or other physical damage.
The major variation in acceleration occurs during the initial period of the propellant burn, generally the first 120 to 175 milliseconds of the burn. This initial period may be termed "catapult stroke". For example, a cold catapult, i.e., a catapult at -65.degree. F. exhibits about the same acceleration during the catapult stroke as during the remainder of the propellant burn, approximately 8 g's. In contrast, for a hot catapult, i.e., a catapult at +150.degree. F., acceleration during the catapult stroke increases to about 14 g's, thereafter decreasing to about 8 g's. The data available are for a standard propelled weight of 400 pounds, i.e., combined weight of ejection seat and weight of the ejectee.
It is known to incorporate temperature compensating means into a rocket motor to insure a constant burning rate regardless of ambient temperature variations. For example, U.S. Pat. Nos. 2,612,747; 2,909,032; and 3,102,383 each disclose temperature responsive means for controlling the effective orifice area of the exhaust nozzle of the rocket motor in a manner to insure constant propellant burning rate.
It is also known to incorporate pressure relief means, as disclosed by U.S. Pat. No. 2,937,830, to vent excessive gas pressure to the atmosphere during the catapult stroke event.
The vertical acceleration experienced by an ejectee during the catapult stroke event is given by a.sub.Z =gF.sub.c /W.sub.e, where a.sub.Z is the vertical acceleration, g is the acceleration due to gravity, F.sub.c is the catapult force and W.sub.e is the ejected weight. It can be seen that acceleration is increased either by increasing the catapult force or by decreasing the ejected weight. Thus, if adequate performance is achieved for a 237 pound ejectee at -65.degree. F., the acceleration will be far too great for a 165 pound ejectee at +150.degree. F. Therefore, what is desired is an ejection system which will limit the vertical acceleration of ejection seats during the catapult stroke event of emergency escape independent of ambient temperature and ejectee weight.
Accordingly, it is an object of the present invention to provide an ejection apparatus having means to limit the vertical acceleration of the ejection seat and its occupant during the catapult stroke event of emergency escape.
Other objects will be readily apparent to those skilled in the art by reading the following description and an examination of the accompanying drawings.